According to NIST and ECRYPT II, the cryptographic algorithms used in
Bitcoin are expected to be strong until at least 2030. (After that, it
will not be too difficult to transition to different algorithms.)

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The MS Windows CryptoAPI source produces cryptographically-strong random numbers even without a hardware RNG. It is designed for exactly this purpose. TrueCrypt uses other sources as well just in case there's some defect in CryptoAPI (and to be assured of similar security properties across platforms), but no defect is known or suspected.

If you'd like to add randomness to the system source, you can easily do so, and TrueCrypt will get it. Just pass it to 'CryptGenRandom' as the auxiliary seed. If the file is large, you may just wish to pass a hash of it. I don't think there is any benefit to doing this, but you certainly can if you wish. Here's the basic code: